(1) Field of the Invention
The present invention relates to a semiconductor device, a process for producing the semiconductor device and a lead frame used in the process for producing the semiconductor device.
(2) Description of Related Art
A general process for producing a semiconductor device includes a die bonding process in which a semiconductor element is bonded on a stage part of a lead frame, a wire bonding process in which each electrode of the semiconductor element is connected to one of the leads of the lead frame by a wire, and an encapsulation process in which the semiconductor is encapsulated by a resin package or a ceramic package. The encapsulation process is carried out after the wire bonding process. Recently, there has been a tendency to increase the number of leads provided in a lead frame and accordingly an increase in the density in the leads of the semiconductor device. Thus, the pitch between adjacent leads becomes small. It is desired that a semiconductor device having a lead frame in which the pitch between adjacent leads is small be produced in high volume.
FIG. 1 is a cross sectional view showing a conventional semiconductor device referred to as a SOP (Small Outline Package) type. In the semiconductor device 1 shown in FIG. 1, a semiconductor element 2 is bonded on a stage part 3 of a lead frame, and each electrode of the semiconductor element 2 is connected to one of leads 4 by a wire (formed, for example, of Au), in a wire bonding process. The semiconductor element 2 and a part of each of leads 4 are encapsulated by a resin package 5. The part of the each of leads 4 located inside the resin package 5 is referred to as an inner lead 8. A part of each of the leads 4 located outside the resin package 5 is referred to as an outer lead 9. The wire is bonded to an end of the inner lead 8, and the outer lead 9 is electrically connected to an external circuit. In the production process of the semiconductor device, at least until the encapsulation process is finished, the stage part 3 and the leads 4 are integrated, as a lead frame, with each other.
FIG. 2 shows a conventional lead frame. A lead frame 6, shown in FIG. 2, is constituted by a stage part 7, leads each having an inner lead 8 and an outer lead 9, tie-bars 10 each connecting each pair of adjacent leads and a cradle 11 connecting ends of the outer leads 9. The leads are prevented, by the tie-bars 10 and the cradle 11, from separating from each other. The tie-bars 10 are used as location bars for maintaining a pitch of adjacent leads at a predetermined value P in the production process of the semiconductor device. The tie-bars 10 also function to prevent resin from leaking toward the outer lead 9 when a resin molding is performed in the encapsulation process. In FIG. 2, the resin package 5 is located on an area A.sub.p. In the encapsulation process, the lead frame 6, obtained after the die bonding and wire bonding processes, is set in a die 12 (constituted by an upper die 12a and a lower die 12b) as shown in FIG. 3. When the lead frame 6 is set in the die 12, a small space A is formed between the upper die 12a and the lower die 12b in a region between adjacent inner leads 8. In this condition, when the resin molding is carried out in the dies 12, the resin is leaked from the dies 12 via the small space. However, the leaked resin is trapped in a space surrounded by each tie-bar 10 and each pair of adjacent inner leads 8.
After the encapsulation process is finished, the tie-bars 10 and the cradle 11 are cut off so that the leads 4 project from both sides of the resin package 5, as shown in FIG. 4. After this, the leads 4 are bent and dipped in solder, so that the semiconductor device 1 is shaped as shown in FIG. 1.
Due to cutting of the tie-bars 10, a cutting part 4a having a new shear plane exists on each side of each of the leads 4. As the lead frame 6 is strained by heat supplied in the resin molding process (the encapsulation process), it is hard to accurately cut off the tie-bars so that a surface (the new shear plane) of the cutting part 4a and the side surface (an original shear plane) of each of the leads 4 are uniformly continued. Thus, in the conventional semiconductor device, the cutting part 4a projects from the side surface of each of the leads 4, as shown in FIG. 4. In a case where the integration density of the leads of the semiconductor device is increased, the number of leads 4 projecting from the resin package 5 is increased. Thus, the projection of the cutting part 4a of each of the tie-bars 10 prevents the integration density of the leads of the semiconductor device from being increased. In addition, the leads 4 are bent so as to have a predetermined shape. However, where a cutting part 4a projects from each lead 4, stiffness of each lead 4 is non-uniform. As a result, each lead 4 is difficult to bend in a predetermined shape, so that there are cases where adjacent leads 4 can not be in contact with each other and where the lead is not connected to the external circuit. On the other hand, when the length of each tie-bar 10 to be cut off is lengthened to prevent a cutting part 4a from projecting from each side of each lead, a case where the width of the lead at the cutting part 4a is less than the with of the lead at a part having no cutting part 4a can occur. In this case, as the width of the lead at the cutting part 4a is smaller the strength of the each of leads 4 is decreased.